Interpretive Summary: Abortions in cattle caused by consumption of needles from Ponderosa pine trees (PN) are a significant risk in the Western United States. This risk is determined by a combination of the risks of consumption of PN and of an effect once PN are consumed. Risk of abortions once PN have been consumed is affected by amount consumed and stage of pregnancy. We assumed that low temperature is one of the primary risk factors affecting consumption. Previous data on risks associated with stage of pregnancy were used to determine the regression of day of gestation when PN are eaten on the probability (PD) of abortions. This regression was: PD = -88 + .74 d with 0 PD being from 0 to 120 d and 1.0 PD being from 255 d through calving. A 107-year data set (1893 to 2000) for Miles City, MT, was obtained from the National Weather Service to determine the probability of temperatures being Technical Abstract:
Abortions in cattle caused by consumption of needles from Ponderosa pine trees (PN) are a significant risk in the Western United States. This risk is determined by a combination of the risks of consumption of PN and of an effect once PN are consumed. Risk of abortions once PN have been consumed is affected by amount consumed and stage of pregnancy. We assumed that low temperature is one of the primary risk factors affecting consumption. Previous data on risks associated with stage of pregnancy were used to determine the regression of day of gestation when PN are eaten on the probability (PD) of abortions. This regression was: PD = -88 + .74 d with 0 PD being from 0 to 120 d and 1.0 PD being from 255 d through calving. A 107-year data set (1893 to 2000) for Miles City, MT, was obtained from the National Weather Service to determine the probability of temperatures being <_-10 deg C (PT). Daily PT were averaged for bimonthly (1st and 15th) intervals and were 0 from April 15 through Oct. 15 and then were .05, .19, .33, .45, .55, .56, .59, .49, .39, .19, and .07 from Nov. 1 through April 1. Cows were simulated to calve at 24 bimonthly intervals (1st and 15th). Daily PT were multiplied by the PD curve of an abortion and summed across days to obtain a cumulated relative risk (CRR) by calving date. CRR was 1 for cows calving Nov. 1 and 4, 9, 15, 23, 31, 39, 42, 45, 43, 39, 33, 27, 20, 15, 10, 6, 4, 2, 1, 0, 0, 0, and 0 for each of the 23 subsequent bimonthly periods. CRR was high for cows calving from Jan. 15 through May 1 with maximal risk on March 1. We conclude that date of calving may be used as a tool to decrease risk of PN induced abortions in cattle with minimal risk occurring for cows calving from June 1 through Dec. 15.